Bifoldable instance for Map

[Joseph C. Sible]

Aren't you basically just saying that you lose some of the structure
(namely, the knowledge that the "key" and its "value" go together)?
But doesn't every Foldable instance on a type that's more complex than
a list also do that? For example, if you fold a rose tree, you lose
the knowledge of which elements came from which branches.

(Completely unrelated: "Loost" and the names of its data constructors
sound like something straight from Dr. Seuss.)

Joseph C. Sible

On Mon, Jun 1, 2020 at 8:28 AM David Feuer <david.feuer at gmail.com> wrote:
>
> Let me be more specific. Whereas we can get intuition for Foldable from
>
>     toList :: t a -> [a]
>
> we get intuition for Bifoldable from the hypothetical
>
>     toEitherList :: t a b -> [Either a b]
>
> This seems quite reasonable for some types.
>
>     data Loost a b
>       = Nool
>       | Corns b (Loost a b)
>       | Colns a (Loost a b)
>
> But for something like
>
>     newtype Plist a b
>       = PNil
>       | PCons a b (PList a b)
>
> it feels awfully strange. Independent parts of the structure just get lumped together.